Elucidation on Performance and Structural Properties of Skinned Asymmetric Nanofiltration Membrane Based on Theoretical Models

In this study, the performance and structural properties of enhanced skinned asymmetric nanofiltration (NF) membranes were experimentally and theoretically analyzed. Based on Donnan and steric-hindrance transport mechanism, the relationship of performances and key properties of the fabricated nanofiltration membranes were examined. At the optimum concentration of polymer, the skinned nano-filtration membranes achieved high salt rejection up to 85% and high solutes separation efficacy. Moreover, morphological and modeling analysis discovered that, the optimum membranes produced good pore size and fine key properties with 1.20 nm of pore radius, 4.96 µm of ratio of thickness to porosity (∆ x/A k ) and −1.56 of surface charge, ζ as well as uniform pore size distributions. The findings from this study proved that the strategic utilization and manipulation of good membranes material is a simple and good attempt to upgrade the membranes capability and usability which lead towards the application in various membrane separation


Introduction
Membrane technology has gained widespread attention for various separation applications such as from microfiltration, ultrafiltration, nanofiltration, reverse osmosis and also gas separation.There are several procedures that can be used in membranes fabrication such as phase inversion, non-solvent induced phase separation (NIPS), and thermal-induced phase separation (TIPS) [1].
Phase inversion technique is the most commonly used in making asymmetric membranes.Phase inversion is initiated as immersion precipitation occurred when polymer is cast onto a suitable substrate and then submerged in a coagulation fluid.The interchange of solvent and non-solvent in phase inversion will generate an asymmetric membrane with a denser top layer and also have very thin more or less dense skin reinforced by a porous sub-layer [2,3].
A skinned, thin and dense asymmetric nanofiltartion (NF) membrane could be prepared by using dry/wet phase inversion technique [4,5].In this technique, the formation of fine skinned membranes depends on two major factors of kinetic and thermodynamic factors which including of interactions between solvents and polymers, interactions between sol-vents and non-solvents, interfacial stability, solvent de-mixing rate [6][7][8][9].Polymers like polyvinylidene fluoride (PVDF), cellulose acetate (CA), polysulfone (PSf), polyethersulfone (PES) have been used for the making asymmetric membranes due to their good thermal stability, outstanding oxidative and good mechanical strength [10,11].
In addition, the used of different membranes materials enabling the making of membranes with a wide range of varied pore sizes that maybe used in numerous of modules and configurations for many industries like gas separation, water purification, medicinal treatments, wastewater treatment and biotechnology [12][13][14].
1.1 Nanofiltration membranes NF membranes are utilized not only to separate aqueous solutions containing electrolytes but also for separation of uncharged solute with size range of about 1-10 nm.Basically, NF membranes are used for separation of solutes and salts based on their unique properties.Besides, it also used as a pre-treatment in the desalination process as NF allows diffusion to occur on certain monovalent ions such as sodium and chloride.Thus, for more complex molecules and multivalent ions are highly retained [15].
Rapid growth on industrial interest and demand for NF membranes are driven by the dual factors of steric hindrance and Donnan factors as transport mechanism for the efficient separation capability [16][17][18][19][20]. NF has been widely used in applications namely in the production of drinking water and also water treatment.NF has been applied in treatment of water including ground, surface and effluent water because of its ability to remove organic and particle impurities [21].
According to the recent literature, the NF process has been applied as a new approach in agricultural manufacturing process, focusing on concentrating plants extract for commercial use in pharmaceutical and food industries [22].

Transport mechanism and modeling
In generally, NF membranes possess of negative charge, and the separation process will be occurred in according to the different magnitudes of signs and valences for the different membranes process separation performance under specific assumptions which derived by the established irreversible thermodynamic model [4,23].A steric hindrance factor actually is a transport mechanism which could be explained as the comparison of solutes size and membranes pores.The larger solutes will be rejected and allow the smaller ones to be permeated easily through the membranes.Meanwhile, the Donnan effect referred to the charge interactions of ions and the membrane [1].
Therefore, transport models were applied to identify the structural characteristics, electrostatic characteristics, and transportation processes of NF membranes.These models are relied on mass transport mechanisms such as diffusion, adsorption, concentration polarization, ion exchange or others.So each model has been developed for a particular set of circumstances.Based on steric hindrance pore (SHP) model and Teorell-Meyer-Sievers (TMS) model, certain presumptions have been made.For example, these models are used to define partitioning effects as membrane is assumed a charged porous layer.Besides, the extended Nernst-Planck equation is used to calculate mass transfer through the membrane.These presumptions will be useful to calculate a membrane's attributes [24].In addition, the data on the transport of a solvent or solute via NF membranes can be evaluated using the Spiegler-Kedem model [25].
Based on the previous reports, these SHP, TMS and Spiegler-Kedem theoretical models were applied in the experiments to determine the optimum performance of NF membrane in terms of key properties like pore radius (r p ), surface charge (ζ ) and pore size distribution [26,27].Furthermore, TMS model also applied to calculate the reflection coefficient, (σ) in the single salt solution experiments.This is important in order to investigate the rejection behavior of salt solution [28,29].
It is well recognized that the skin active layer determined the amount of solutes permeate from the membranes.This transport of these solutes was driven by the characteristics of membranes morphology, pore size, charge density and skin active layer [30].The ions separation can be revised by the improvement in membrane efficacy [31].
In addition to design factors, fabrication parameters are also crucial to the characteristics of the produced membrane [1].For example, the influence of PES as polymer concentration in fabrication of membrane led to the inducing of chain entanglement and therefore reduced the microvoid concentration in the skin layer [32,33].Therefore, the rights choose on materials, including polymers, solvents, and non-solvents, when creating asymmetric membranes really need to be considered [34].
As the polymer concentrations factors had been reported affect the membranes separation performance, its significant effects on the membranes properties and structural details are not fully evaluated.Moreover, the use of modeling technique and pore size distribution analysis to explain the factor of polymer concentrations on membrane performance-properties is not fully addressed.Therefore, the relationships between the composition of polymer material and performance-properties induced by enhanced membranes parameters and structural details of skinned NF membranes were experimentally studied and modeled.

Experimental 2.1 Materials
The polyethersulfone (PES, Radel A300) and N-methyl-2-pyrrolidone used as raw materials.Methanol and water were used as a post-treatment media.For the solute separation test, the neutral solutes of glycerol, glucose, saccharose, raffinose and PEG 1000 (Sigma Aldrich) were employed [35].Besides, monovalent salt (NaCl) and multivalent salts (MgCl 2 , Na 2 SO 4 and MgSO 4 ) were used to evaluate the membranes performance.

Dope formulations and membrane making
A polymer/solvent/non-solvent ternary dope component was carried out through a turbidimetric titration method at room temperature and 84% humidity [8].In this method, 100 g of polymer solution was titrated with distilled water until the dope converted into a milky solution.Then, the dope formulation was measured as tabulated in Table 1.Based on these formulations, the membranes dopes (500 ml) were prepared using dope apparatus.
Prior to casting, the membranes dope was then placed into ultrasonic bath for 6 hours to remove air bubbles and degassing purpose.Subsequently, the membranes were cast on a glass plate using auto-casting machine at room temperature (30 °C).The membranes thickness was fixed at 150 μm.Then, the membranes were dipped into water bath (coagulation media) for 24 hours.For the post treatment process, membranes were dipped into n-hexane and ethanol for 6 and 24 hours, respectively.Finally, the treated membranes were dried for 24 hours at room temperature before it ready to be used in the nanofiltration test.

Membrane performance
NF membranes samples were undergo a compaction test at 500 kPa for 1 hour.20 mL of permeate samples were collected for each three independent experiment run and weighed minutely by an electronic balance.The salts concentration in feed and permeate stream were measured using a conductivity meter (Model Hach SENSION5).The solutes rejections were examined for fluxes and real rejection at a pressure of 500 kPa using a Millipore permeation cell with membranes area of about 1.39 × 10 −3 m 2 .The measurements of the neutral solutes were conducted using a Total Organic Carbon (TOC) Analyzer from Thermo Scientific.
In this study, the Scanning Electron Microscopy (JEOL JSM 6360LA) was used to analyze the membranes morphologies.For this analysis, the membrane samples were cryogenically ruptured after being placed in nitrogen liquid for a brief period of time for morphological study.The broken samples were placed cautiously on stubs and covered with a gold layer.The membranes samples were scanned using 600X-2500X magnification and 10.0 kV potentials under a microscope.Moreover, the pore size distribution analysis was conducted using neutral solutes rejection data and MiniTAB ® statistical software [36,37].

Membrane characterizations and modeling 2.4.1 Permeation of fluxes
Permeation of fluxes data were evaluated based on the following equation (Eq.( 1)) [1]:

Real rejection, R real
The NF membranes separation capability was evaluated as follows [38]: (2)

Spiegler-Kedem and Steric-Hindrance Pore (SHP) model
Based on these models, the transports element in NF membranes is mainly driven by diffusion and convection.The separation properties of NF membranes and process are well-described by the following equations (Eqs.( 3) to ( 13)) [38,39].The used of these equations for determining the membranes parameters was described elsewhere as in our previous report [26]: where: The combination of Spiegler-Kedem and SHP model allowed the determination of membranes structural parameters such as reflection coefficient, (σ), pore radius, (r p ), solutes permeability (P s ), ratio of thickness to porosity (∆x/A k ) and the important steric-hindrance parameters (H F , H D , S F and S D ) [26,40].

Teorell-Meyer-Sievers (TMS) Model
This model was used to quantify and describe the electrostatic properties.This model implies the fixed circular dispersion of charges and ions that occurs in membrane electrical characteristics for determination of the Donnan parameters such as reflection coefficient of salt, (σ salt ), salt permeability (P salt ), charge density, (X d ) and the membrane surface charge, (ζ ) [41].The derived equations to estimate the electrostatic properties were previously discussed in our report [26].

Membranes performance and separation capability
In order to fabricate high performances membranes, turbidimetric titration measurement was used to determine the compositions in all cases.The dope solutions' compositions were collated as shown in Table 1.
Variations in the polymer concentration in the dope solution result in various membrane structures with various membrane properties.Additionally, the optimum polymer concentration must be taken into account as demonstrated that at these optimum polymer concentration, dope solution starts to exhibit noticeable chain entanglement.The pure water permeation (PWP) of the prepared NF membranes was depicted as in Fig. 1.
The symbols represent average of three experiments run, in which error bars represented the standard deviation.The error bars are not visible when they are smaller than the size of symbols in the graph as also reported by other researchers [42][43][44].The membranes permeability can be interpreted directly from PWP data whereas the compressibility data can be evaluated based on the water flux changes with times.Meanwhile, membranes volume flux and salt rejection were depicted as in Figs. 2 and 3, respectively.
From Fig. 2, the increasing in operating pressure caused the higher membranes fluxes.At lower polymer concentration of NF19 membranes exhibited higher flux.However, there are not many changes in terms of volume flux between these membranes over the pressure range.At 450-500 kPa, the highest flux of about 4.08 × 10 −6 m 3 /m 2 s was performed by NF19 membrane.This may be the result of the membrane skin layer porosity which is the more porous membranes lead to more productivity.
In comparison, the obtained volume fluxes are found to be comparable to the market available membranes such as NF-45 and AFC 30 (7.0 × 10 −6 m 3 /m 2 s at 500 kPa) [45][46][47].Fig. 3 demonstrates that as pressure increased, more NaCl was rejected.The membranes displayed higher salt rejection at high polymer concentrations (> 20 wt%).Membranes performances in Fig. 2 and Fig. 3 showed that the increasing of polymer in dope solution produced a thicker, denser skin layer that encouraged the development of more selective membranes.In contrast, casting membranes with low polymer concentration produced a thin, porous skin layer that had a high degree of permeability and low separation performances.This outcome was consistent with earlier proved study [48].
Fig. 4 showed the fluxes and salt rejection of skinned membranes.It was clearly observed that there is an intersection of membranes performances closed to the polymer concentration of 20.42 wt%.At this point, the fabricated membrane showed moderate performance.Although the rejection increased, the membranes volume flux was found In Fig. 5, the rejection results of multivalent salts (Na 2 SO 4 , MgSO 4 , MgCl 2 and NaCl) at 500 kPa of operating pressure were plotted.It can been that the increment in polymer concentration affected the rejection performances.In a sequence of rejection performances of MgSO 4 > Na 2 SO 4 > MgCl 2 > NaCl, high polymer concentration (>20 wt%) demonstrated of high salt rejection more than 85%).
Table 2 demonstrates that the solute size followed the order of Mg 2+ > SO 4 2− > Na + > Cl − , whereas the ion diffusivity (D) follows the sequences of Cl − > Na + > > SO 4 2− > Mg 2+ .The trends may assist in comprehending how the real rejection in Fig. 5 was acquire.The solutes transports through the NF membranes are strongly influenced by diffusivity and size.The Donnan concept states that a higher valence counter-ion results in less in salt rejection [49,50].

Membrane parameters and structural properties
In this study, different types of uncharged/neutral solutes of different series of molecular weight, diffusivities  and solutes are listed in Table 3.These solutes are very famous and commonly used as solutes maker by numerous researchers for NF experiment [51,52].In Table 3, the average values of ∆x/A k were determined based on the modeling analysis using Eqs. 9 and 10 which taken into account if the reflection coefficient, σ not more than 0.9 whereby the effective membrane thickness, ∆x was fixed at 150 μm [24,40,53].Table 4 displayed the parameters and characteristics data of the prepared NF membranes.Generally, according to modeling studies the use of solutes with varying molecular weights often increased the reflection coefficient, σ and decreased the solute permeability, P s.The percentage of smaller membrane pores compared to the molecules in the testing solution, enabling for an evaluation of the performance of the membrane, is represented by the value of σ.At the optimum polymer content, the NF21 membranes possess of pore radius, r p and ratio of membrane thickness to porosity, ∆x/A k are in the ranges of 0.96 to 1.37 nm and 4.96 to 98.29 µm, respectively.The smaller and good values of these properties were resulted in increment of salt rejection and good data in water flux.Although the increasing of polymer concentration produced smaller pore radius, the solute permeability and porosity factor were found to be very low.Moreover, at the optimum polymer concentration, the ratio of solute radius over pore radius, η shows good values from 0.19 to 0.82.In details analysis, the optimum polymer concentration produced of good ranges membranes parameters and fine properties which are very important and affected the separation performance and NF transport mechanism [54,55].
Modeling results on the membranes parameters and properties showed that the polymer concentration played a significant roles determining of NF characteristics and separation performance.For further analysis, the impact of polymer content on the steric-hindrance factors of the fabricated membranes at different polymer concentration was summarized in Table 5.The steric effects, S F and S D , were lowered as polymer concentration increased in terms of convection and diffusion factor.At the optimum polymer concentration, the membranes demonstrated of good values of steric-hindrance factors.In combination to a good value of reflection coefficient separation factor, σ = 0.12, these parameters and factors promoted the membranes for a good permeation, high selectivity and fine morphology.In accordance with the SHP model, these parameters greatly influenced the membrane transport mechanism for NF process [55].
In the membranes fabrication and characterization, the changing of membranes parameters and properties are very significant to the membranes separation performance.In order to analysis the fabrication and performance-properties of asymmetric NF membranes, full set of modeling data on membranes properties were determined and tabulated in Table 6.Modeling data of the electrostatic properties showed that the membrane surface charge density, X d and zeta potential, ζ were increased at higher polymer concentration.At low polymer concentration (< 20 wt%) produced small values of electrical properties while the higher polymer concentration the higher values obtained.At the optimum polymer concentration, the membranes discovered to have surface charge density, X d and zeta potential, ζ of about 0.027 and −1.56, respectively.

Morphologies and pore size distribution
Based on images analysis by Scanning Electron Microscopy (SEM) technique, different features of membranes pores and sublayer such as finger like, cellular, sponge like, nodular, compact/dense structure and tears like were found to be affected the membranes performances and separation characteristics.A dense and thicker skin layer was created as the polymer concentration in the casting solution increased, producing an asymmetric membrane that was more selective but less effective [55][56][57].
As the high concentration of polymer displayed thicker skin layers with highly porous substructure, the lower polymer produced thinner skin layers with large uniform voids across the substructure.The findings proved that the membranes performance and characteristics are depending to the features polymer solution and materials interactions and constituents [58,59].Fig. 6 showed the SEM images comprises of cross section structures and skin layers of skinned NF membranes at different PES concentration.These images depicted of asymmetrical structures as well as smooth and even top skin layer.
Based on the SEM images, the NF membranes structures composed of thinner active layer and highly porous finger like sublayer.As increment in polymer concentration, PES membrane frequently develops a thicker and denser skin layer.It showed similar cross sectional morphologies with very porous macrovoids.Fig. 6 (a)-(e) depicts that the length of the macrovoids decreased as the polymer concentration increased.At low polymer concentration, the membranes produced large macrovoid whereas a much smaller macrovoids was formed from high polymer concentration solution.However, at high polymer concentration (> 21 wt%), the NF membranes posed of a narrow like pores and spongy sublayer.The declined in flux and salt rejection are the result of minimal pores distribution and the suppression of macrovoid.
As a result, it was anticipated that the sublayer's demixing and precipitation processes would occur more slowly and resulted towards thick and dense skin layer in asymmetrical membranes [60,61].In addition, the casting solution's viscosity was raised by the addition of more polymers, producing membranes that were more selective but less productive [62].The crucial parameter affecting properties-performance of membrane is the amount of polymer concentration, which directly impacts the viscosity of casting solution [63].
Cross sectional membranes analysis revealed that thicker skin layer thickness was produced at high polymer concentration.The formation of skinned macrovoids with surrounded of thick dense layer instead of finger like structures resulted to a separation improvement.The formation of this skinned macrovoids maybe connected to a decline in demixing rates driven on by viscosity effects [64,65].
Based on the results of solute rejection as represented in Table 4, solute transport method was employed for determination of pore size and pore size distribution (PSD) determination.The nature of membranes pore size distribution was examined based on method described by Bowen and Mohammad [66].In terms of probability density function, the pore size distribution of skinned membranes with difference polymer concentration was depicted in Fig. 7.
From the graph, the membranes showed a good and narrow pore size distribution.It was revealed that at higher polymer concentration the narrower pore size distribution was produced.Hence, high polymer concentration (> 20 wt%) not only improved the membranes performances but also produced good membranes characteristics as well as the narrowest pore size distribution as reported by other researchers [9,18,32].
This finding also could be the significant reason of membranes performances profiles.Besides, the analysis result also support and prove that the polymer concentration played an important role determining of separation performance and membranes properties.Meanwhile, Fig. 7 showed the summary of NF membranes morphological structures-pore size distribution and morphological structures-key properties, respectively.Pore size distribution analysis and modeling results revealed that the NF21 membranes produced fine pore size distribution and key properties with 1.20 nm of pore radius, 4.96 µm of ∆x/A k and −1.56 of surface charge, ζ, respectively.This data proved that the optimum polymer concentration for this study was found to be at 20.42 wt%.

Conclusions
The polymeric skinned asymmetric NF membranes were successfully developed.Analysis on the performances-properties showed that the increasing of polymer concentration in dope solution produced selective membranes.At an optimal PES concentration, the skinned NF  4 membranes possess of good performance, fine structural and key properties.The nanofiltration membranes produced at 20.42 wt% possess the narrow of pore radius and high surface charge of about 1.20 nm and 1.56 respectively.Based on modelling analysis, polymer concentrations factor was found to be significant and practical for the modification of membrane performance and properties.The findings from this study also proved that the manipulation of good membranes material is a good attempt to upgrade the membranes characteristics and efficacy which are potentially to be applied in specific applications such as in water treatment and wastewater treatment industries.

Acknowledgment
The authors gratefully acknowledge of ESERI and INOS for their assistance and advice.

Fig. 1
Fig. 1 PWP of skinned NF membranes.Error bars represent the standard deviation and not shown when they are smaller than size of symbols

Fig. 2 Fig. 3 Fig. 5
Fig. 2 Volume fluxes of 0.01 M NaCl.Error bars represent the standard deviation and not shown when they are smaller than size of symbols

Table 1
Multi components dope solution formulations

Table 3
Membranes parameters and properties of skinned NF membranes at different PES concentration

Table 4
SHP parameters for skinned NF membranes at different PES

Table 5
Properties of skinned NF membranes

Table 6
Commercial NF vs skinned NF membranes * Mean values of modelling data